The rodent in hibernation has two reactions when exposed to low ambient temperatures. Between body temperatures (Tb) of about 3 degrees to 9 degrees C with relatively rapid chilling, the animal may respond at a definite body temperature (Tset) by increasing heart rate, respiratory rate, oxygen consumption, and by muscular activity (Reaction I). When chilled very slowly, a response occurs at a body temperature (Tb) of 1.5 - 2.5 degrees C (Reaction II). In this latter response there is no muscular activity, and threshold is in a very narrow range of Tb. Reaction II will be further defined by obtaining brain temperatures when the reaction occurs, and by observing the effect of preoptic hypothalamic lesions. Since no shivering occurs at the outset, the putative role of brown fat thermogenesis is being explored by 1) measuring temperature changes in brown fat pads, 2) comparing the temperature of each axillary region when brown fat has been removed from one side, 3) comparing the thermogenesis of the unoperated animal in hibernation with the same animal after near total ablation of brown fat. All hibernating rodents become more sensitive to external stimuli as a bout of hibernation continues, but only the Turkish hamster is incapable of arousing on its first day of hibernation. We have shown that arousal will occur if the animal is first infused intraperitoneally with small amounts of epi- or norepinephrine. The effect of catecholamines on progressive irritability is being clarified during a bout by defining precisely the amount of epi- and norepinephrine necessary to produce these results when infused intra-arterially. The change in the concentration of these amines in the blood during a bout of hibernation will be determined.